1. Field of the Invention
The present invention relates to a droplet ejection device such as an inkjet head.
2. Discussion of Related Art
As a droplet ejection device, there is known an inkjet head operable to eject an ink droplet. U.S. Pat. No. 6,648,455 (corresponding to JP-2002-59547A) discloses an inkjet head including: (a) a cavity unit having a plurality of nozzles and a plurality of pressure chambers held in communication with the respective nozzles; (b) a piezoelectric actuator unit including a plurality of actuators operable to change a volume of a corresponding one of the pressure chambers; and (c) a flexible flat cable for supplying a drive voltage to each of the actuators.
In the inkjet head disclosed in U.S. Pat. No. 6,648,455, the plurality of pressure chambers are disposed in a rear portion of the cavity unit, and the piezoelectric actuator unit is superposed on the rear portion of the cavity unit such that the plurality of actuators correspond to the respective pressure chambers. The piezoelectric actuator unit includes (i) four individual-electrode layers each having a plurality of electrode layers which correspond to the respective pressure chambers and which are electrically independent from each other, (ii) three common-electrode layer each having a common electrode which is common to all the pressure chambers, and (iii) six piezoelectric layers each made of a piezoelectric material. The individual-electrode layers and the common-electrode layers are alternately arranged, with each of the piezoelectric layers being interposed between a corresponding adjacent pair of the individual-electrode and common-electrode layers. Each of the piezoelectric layers has a plurality of deformable portions each of which is interposed between the individual and common electrodes. Each of the deformable portions is polarized in a thickness direction of the piezoelectric layer. With application of a drive voltage between a selected one of the individual electrodes and the common electrode (that is grounded), a corresponding one of the deformable portions of the piezoelectric layer is caused to expand or contract in the thickness direction, thereby changing a volume of a corresponding one of the pressure chamber that are filled with an ink. As a result of change of the volume of the corresponding pressure chamber, the ink droplet is ejected through the nozzle held in communication with the corresponding pressure chamber.
In the disclosed inkjet head, the four individual-electrode layers and the three common-electrode layers are alternately arranged with each of the six piezoelectric layers being interposed between the corresponding pair of the individual-electrode and common-electrode layers, so that a total of six deformable portions correspond to each of the pressure chambers. For causing the ink to be ejected from a selected one of the pressure chambers, the drive voltages are concurrently applied to the four individual electrodes corresponding to the selected pressure chamber, whereby the six deformable portions are concurrently caused to expand. In this instance, the volume of the selected pressure chamber is changed by an amount corresponding to a sum of amounts of the expansions of the six deformable portions Meanwhile, the drive voltage is not applied to any one of the four individual electrodes corresponding to a non-selected one of the pressure chambers (from which the ink is not be ejected), so that the six deformable portions corresponding to the non-selected pressure chamber dot not expand at all, whereby the volume of the non-selected pressure chamber is not changed. It is noted that there is also known an arrangement in which the volume of each pressure chamber is changed by causing the corresponding deformable portions to concurrently contract rather than expand.
In the disclosed inkjet head in which the individual electrodes are arranged to correspond to the respective pressure chambers and to be electrically independent from each other, there are required the same number of individual electrodes as that of the pressure chambers, i.e., that of the nozzles. For example, where the inkjet head has the nozzles arranged in a matrix consisting of seventy-five rows and two columns, a total of 150 outputs have to be provided in a controller circuit operable to apply the drive voltage to each of the electrodes. That is, for merely applying the drive voltage to each electrode, the 150 outputs are required. Such a large number of the required outputs leads to an increase in cost required for the controller circuit. Further, since the inkjet head (to which the large number of outputs are to be supplied via the flexible flat cable) by a small-sized device, a large number of output lines have to be arranged in a high density within the flexible flat cable.
Still further, there is a tendency of increase in the number of nozzles provided in each head, for attending a need for recording or printing a higher density of full-color image at a higher speed. In the above-described conventional arrangement of the electrodes, if the number of columns is doubled to four, the required number of outputs of the individual electrodes is doubled to 300, whereby the costs required for the controller circuit and flexible flat cable are inevitably increased.